1. Field of the Invention
The present invention relates to an electric power steering control apparatus that serves to provide an assist force generated by the rotational force of an electric motor (hereinafter referred to simply as a “motor”) to a steering system (a steering wheel) of a vehicle, and more particularly, it relates to a new technique for reducing the driving control sound of the motor.
2. Description of the Related Art
In the past, there have been proposed a variety of electric power steering control apparatuses in which the steering torque of a steering system (a steering wheel) is detected and an assist torque to a steering system is generated by a motor by controlling the drive current of the motor in accordance with the detected value of the steering torque.
In case of such a kind of electric power steering control apparatus, it is known that a control sound is generated from the motor upon variation of a motor current command value.
In general, as a cause for the control sound generated from the motor, there can be considered vibration due to time harmonic of a power supply, vibration due to an unbalance of a rotor of the motor, etc., and such a vibration component is amplified in particular by the unbalance of the motor to generate discordant or harsh noise offensive to the ear.
Accordingly, there have been proposed a variety of electric power steering control apparatuses that serve for the purpose of preventing control sounds from being generated from a motor (see, for example, a first patent document (Japanese patent application laid-open No. H5-301578)).
In a conventional apparatus as described in the first patent document, when a motor current command value is calculated which serves to supply a steering system with an assist torque corresponding to a steering torque applied to a steering wheel by a driver, a comparison is made between the current detected value and the last (one sampling before) detected value of steering torque detection signal sampled in a time series manner (in a period of 1 msec) so as to calculate an average value, which is then set as a motor current command value. As a result, the variation of the motor current command value is suppressed, and the variation of an actual motor current is also suppressed, thus preventing the generation of a control sound from the motor.
In the conventional electric power steering control apparatus, as described in the first patent document, upon calculation of the motor current command value, an average value is obtained between the current value and the last value (one sampling before) of the steering torque that are detected by a predetermined sampling period (1 msec). As a result, the actual motor current command value varies in a period of 1 msec, too, so the control sound of the motor will change at a frequency of 1 kHz.
However, since the diapason or frequency of 1 kHz is a high level frequency (loudness level) that is particularly felt as unusual noise to man, there has been a problem that when the variation of the motor current command value is large, the unusual noise of 1 kHz generated from the motor can not be suppressed to a satisfactory extent.
On the other hand, it can be considered that the processing period of the entire control (sampling period) is set shorter than 1 msec so that the variation of the motor current command value can be suppressed, and at the same time the control sound of the motor is shifted to the side of higher frequencies that man can not hear easily, thereby reducing the level of the unusual noise, but in this case, the calculation load of a CPU is increased due to an increase in the number of samplings, so there arises a problem that it is impossible to realize the control with the use of an inexpensive CPU, thus inviting an increase in cost.